Method and modifying body for influencing the effect of X-ray or gamma radiation on a target sensitive to the radiation

ABSTRACT

Method and modifying body for influencing the effect of X-ray or gamma radiation on a target (7) sensitive to radiation, in particular for selective modification of a radiograph of an object (10). According to the invention a modifying body comprising at least two layer groups (31, . . . 3n) is arranged in front of the target, wherein each layer group (e.g., 32) emits a secondary radiation under the influence of the X-ray or gamma radiation or the secondary radiation of the previous layer group (e.g., 31), respectively, the energy of which is lying above the absorption energy level defined by the electron shell K of an element being present in the following layer group (e.g., 33) or in the target arranged behind the last layer group (3n), respectively.

TECHNICAL FIELD

The invention relates to a method for influencing the effect of X-ray orgamma radiation on a target sensitive to the radiation, preferably forselective modification of an image of an object formed on a target byX-ray or gamma radiation, as well as to a modifying body forimplementing the method according to the invention.

BACKGROUND ART

For producing radiographs, amplifying foils are known which convert theincident X-ray radiation into a radiation falling in the range ofvisible light. In general, these amplifying foils used to be arrangedbefore and behind the film to be shot. In this case blackening of thefilm is primarily caused by the radiation falling into the visiblerange. Application of amplifying foils enables reduction of exposuretime, however, it is accompanied by a loss in the sharpness of theimage.

From the PCT Application PCT/HU83/00062 published under the number WO84/02399 a method for producing radiographs is known, in which amodifying body of laminar structure is placed between the object to beexamined and the film. The modifying body consists of thin Pb layersarranged on one another, which are applied onto a carrier sheet. If inthe object examined material thickness corresponding to different raypaths is changing, e.g. the object is a pipe, the modifying bodyautomatically acts as a compensator. In such a manner, on the image thusobtained far more details of the object can be seen and evaluated,however, the exposure times have to be increased.

In radiologic practice materials of the films used for preparingradiographs respond only to an X-ray or gamma radiation, the energy ofwhich surpasses a given limit of energy level. In general, sensitivityis the highest immediately above said energy level, thereafter withincreasing energy it drops considerably. Just in order to eliminate thisdependency on energy, sensitivity of usual films used to be modified bymeans of filters so as to achieve nearly equal sensitivity in a broaderrange of energy. This, however, involves a poor sensitivity of the filmin the whole energy range, requiring an increased radiation intensity ora longer exposure time in preparing radiographs.

DISCLOSURE OF INVENTION

The aim of the invention is to develop a method and means, by the aid ofwhich an image formed by X-ray or gamma radiation can be advantageouslyinfluenced so that quality of the image could be improved withoutprolonging the exposure time, and if possible, even with a shorterexposure period.

Accordingly, the invention relates to a method for influencing theeffect of X-ray or gamma radiation on a target sensitive to theradiation, preferably for selective modification of an image of anobject formed on a target by X-ray or gamma radiation, in the course ofwhich a modifying body having laminar structure is arranged in front ofthe target in the path of the radiation beam. The improvement accordingto the invention is that in front of the target at least two layergroups are disposed, each of said layer groups comprises a plurality ofsuperimposed layers emitting a secondary radiation under the influenceof the primary radiation or the secondary radiation of the previouslayer group, respectively, the energy of which is lying above theabsorption energy level defined by the electron shell K of an elementbeing present in the following layer group or in the target arrangedbehind the last layer group, respectively.

By proper selection of the material of said layer groups it can beachieved that at least a part of the primary X-ray or gamma radiation isconverted gradually within the layer groups into a radiation with anenergy range in which the sensitivity of the target is maximal. By thebetter utilization of the target a reduction of exposure time becomespossible. The target can be a film being sensitive to X-ray or gammaradiation or a screen or any image-sensing means, e.g. a NaI or CsIcrystal detector doped by Tl.

It has also been recognized that it can be advantageous, if a layergroup is disposed in front of the target, with which the energy of thesecondary radiation emitted by said layer group is lying between theabsorption energy level and the emission energy level defined by theelectron shell K of an element being present in the following layergroup or in the target. By means of this secondary radiation thefollowing layer group or the target can be pre-excited, accordingly, itwill be more sensitive to the radiation with an energy lying above theabsorption energy level.

In accordance with the invention, material and location of the layergroups are to be selected expediently so that when progressing in adirection toward the target, the successive layer groups should containelements with ever decreasing atomic numbers.

Sharpness of the radiograph thus obtained can be increased if at thelayer groups an electric field with a direction corresponding oropposite to the radiation beam is generated, e.g. so that a propervoltage is applied onto the insulated Al layers formed on the twoextreme boundary surfaces of the layer groups lying above one another.In such a manner electrons generated by the X-ray or gamma radiation canbe well controlled.

For absorbing scattered radiation arriving from the object to beexamined and from the environment it is expedient to dispose Pb layersbetween the object tested and the first layer group facing the object.In order to absorb re-scattered radiation it can be advantageous todispose Pb layers lying on one another behind the target.

The invention also relates to a modifying body for influencing theeffect of X-ray or gamma radiation on a target sensitive to suchradiations, said modifying body comprises a plurality of superimposedlayers. The modifying body according to the invention comprises at leasttwo superimposed layer groups, each layer group comprising a pluralityof superimposed layers containing an element which emits a secondaryradiation under the influence of the X-ray or gamma radiation or thesecondary radiation of the previous layer group, respectively, theenergy of which is lying above the absorption energy level defined bythe electron shell K of an element being present in the following layergroup or in the target, respectively.

It is considered as advantageous for the sake of the aforementionedpre-excitation that the modifying body comprises a layer group whichcontains an element emitting a secondary radiation, the energy of whichis lying between the absorption energy level and the emission energylevel defined by the electron shell K of an element being present in thefollowing layer group or in the target, respectively.

Advantageously, layer thickness in the modifying body according to theinvention is less than 100 μm, expediently less than 30 μm. Mostadvantageously the thickness lies in the range between 0.1 and 10 μm, inthis case namely the radiation is compelled to travel through aplurality of boundary surfaces, which involves an increased probabilityof interactions and thus the increase of absorption of the primaryradiation.

In case the Compton-effect dominates in the modifying body, it can beadvantageous to use one or more layers or coatings consisting ofgraphite or carbon or a carbon compound in the modifying body.

It can be expedient, if the surfaces of the layers of the modifying bodyare provided with substantially monoatomic coatings which reduce theescape energy of electrons, consisting of alkali metal or alkali-earthmetal or the oxides thereof or any other compound thereof. A positiveion reduces the potential barrier of the constituent element of thelayer, representing the pre-requisite for the tunnel effect. Evaporationof the coating is not to be feared, accordingly, Li₂ O . . . Cs₂ Ocompounds, as well as oxides of alkali-earth metals are well suitablefor this purpose. One has to reckon also with a secondary electronemission, for which purpose the coatings are well-suited, too. Due tothe secondary electron emission, smoothness of surfaces is imperative,otherwise the electrons are easily trapped on the porous surface.

In respect to the arrangement of the modifying body in an electric fieldit is advantageous, if there is at least one layer made of a conductivematerial--preferably of Al--being insulated from the other layers andprovided with an electric contact and a terminal connected thereto.

The modifying body according to the invention can be prepared so thatall layers of a layer group are applied onto a carrier sheet beingsubstantially transparent for X-ray or gamma radiation, and between thelayers separating layers are inserted. According to another embodimentall the layers of all layer groups are applied to one single carriersheet one after the other. The carrier sheet can be made of paper orplastic or aluminium foil. It is considered as very advantageous, if allthe layers as well as the separating layers inserted inbetween--e.g.made of Al₂ O--are applied onto a single carrier sheet by vacuumdeposition. In the same manner, most easily, eventual coatings on thelayers can be applied also by evaporation.

If an AgBr film is used as a target, from the point of view ofsensitivity, expediently a modifying body is to be selected, with whichthe energy of the secondary radiation emitted by the element of the lastlayer group surpasses by 100% at most, preferably by a maximum of 50%,the absorption energy level of the element of the target.

BRIEF DESCRIPTION OF THE DRAWING

The invention will be described in detail by means of preferredembodiments serving as examples, with the aid of the drawing enclosed,wherein:

FIG. 1 is a schematical view of the modifying body according to theinvention and the application thereof according to the invention, and

FIGS. 2 to 3 are schematical views of embodiments of the modifying bodyaccording to the invention being suitable for an X-ray film target.

MODES FOR CARRYING OUT THE INVENTION

In FIG. 1 a radiation beam 9 emitted onto an object 10 to be examined bya radiation source 1, which can be a usual industrial X-ray apparatus,is confined by an aperture 2. Due to the radiation beam 9 travellingthrough the object 10 radiograph of the object 10 appears on the target7, e.g. on an X-ray film. The modifying body according to the inventionis arranged between the object 10 and the target 7, in front of thetarget 7. The modifying body consists of n layer groups 31, 32, 33, . .. 3n which are superposed on one another and are lying approximatelyperpendicularly to the radiation beam 9. Every layer group is composedof a plurality of layers superposed on each other, so e.g. the layergroup 32 contains k layers 81, 82, . . . 8k. Adjacent layers e.g. 81 and82 are separated by a thin separating layer (not shown). For the sake ofvisibility in FIG. 1 layers and layer groups are not illustratedaccording to the actual scale, and similarly, FIGS. 2 and 3 are notaccurately scaled, either.

If the target 7 is an AgBr X-ray film being sensitive to X-rayradiation, materials of the layer groups 31, 32, 33, . . . 3n areselected so that the energy of the X-ray beam entering into the firstlayer group 31 will be transposed gradually by the layer groups into therange of maximum sensitivity of the X-ray film forming the target 7.Hereinafter, before symbols of the elements the atomic number Z willalways be indicated. The energy level of absorption defined by theelectron shell K of an element, representing the limit value ofexcitation of said element, expressed in unit keV, will be indicatedwith K. The most probable energy levels of emission corresponding to thetransition between the electron shells K and L of the excited element,expressed also in unit keV, are marked with α1 and α2. For theaforementioned gradual energy transposition the constituent elements ofthe layer groups 31, . . . 3n for the AgBr X-ray film can be selectedaccording to the following columns:

    ______________________________________                                        44Ru          60Nd                                                            K = 22.12     K = 43.57                                                       α1 = 19.28                                                                            α1 = 37.36                                                α2 = 19.15                                                                            α2 = 36.84                                                ↓      ↓                                                        41Nb          55Cs                                                            K = 18.99     K = 35.98                                                       α1 = 16.61                                                                            α1 = 30.97                                                α2 = 16.52                                                                            α2 = 30.62                                                ↓      ↓                                                        38Sr          51Sb                                                            K = 16.11     K = 30.49                                                       α1 = 14.16                                                                            α1 = 26.36                                                α2 = 14.10                                                                            α2 = 26.11                                                ↓      ↓                                                        35Br          47Ag                                                            K = 13.47     K = 25.51                                                       α1 = 11.92                                                                            α1 = 22.16                                                α2 = 11.88                                                                            α2 = 21.99                                                ______________________________________                                    

As it becomes obvious from the columns, the elements are to be selectedso that the energy of the exciting radiation falling onto the elementshould surpass the energy level K of absorption, at the same time theemission energy levels α1 and α2 of the characteristic radiation emittedby the excited element should be higher than the absorption energy levelK of the following element. The series of excitation of 35Br prior to44Ru can be as follows: 74W--68Er--62Sm--57La--52Te--48Cd. The series ofexcitation of 47Ag prior to 60Nd can be as follows: 78Pt--71Lu--65Tb. Inrespect to the practice here the series can be closed.

Limit energy level of sensitivity of AgBr X-ray film is defined by theabsorption energy level K of 47Ag. However, we found that it isadvantageous if energy of radiation falling onto the X-ray film isfitted also to 35Br, e.g. in accordance with the previously described35Br series, as in such a manner efficiency of blackening of the X-rayfilm can be increased.

FIG. 2 illustrates a modifying body with such a layer arrangementwherein in the path of the radiation beam arriving in the direction asindicated by arrow 17 a layer group 41 containing layers with 51Sbmaterial is arranged first, followed by a layer group 42 containinglayers with 38Sr material. Emission energy levels α1 and α2 ofcharacteristic radiation emitted by 51Sb are on the one hand fitted tothe absorption energy level K of the component 47Ag of the AgBr X-rayfilm i.e. to the target 7, and on the other hand higher than theabsorption energy level of 38Sr. As a consequence, the latter will beexcited. Energy levels α1 and α2 of characteristic radiation emitted by38Sr are matched to the absorption energy level K of 35Br. Accordingly,the layer group 42 containing layers with 38Sr material allows to pass apart of the incident radiation to the target 7, while the other part isabsorbed and emits instead a radiation with an energy matched to 35Br.

In addition, the modifying body according to FIG. 2 is provided with twographite coatings 15 and 16, which are--with the embodiment describedhere--applied onto the first layers of the layer groups 41 and 42.Graphite coatings 15 and 16 contain 6C with a low absorption energylevel K, behaving as an electron donor. Layers of the layer groups 41and 42 can be additionally provided with a mono-atomic coating (notshown) reducing the escape energy of electrons. Layer groups 41 and 42are surrounded by layers 11 and 12 of Al-material, serving as plates forgenerating an electric field, and they are connected with terminals 13and 14 via contacts not illustrated here. By insertion of an insulatinglayer, the layers 11 and 12 can be applied onto the layer groups 41 and42 by evaporation.

In the modifying body according to the invention, from the series ofelements having been described in connection with FIG. 1, one or moremembers may be left out. In the course of transposition of energy levellarger steps are also permitted.

In FIG. 3 a modifying body for an AgBr X-ray film target 7 can be seen,in which in the path of the radiation beam coming in the direction asindicated by the arrow 17 layer groups 61, 62, 63 and 64 are arrangedone after the other. Layers of the layer group 61 contain 51Sb or itscompound, layers of the layer group 62 contain 50Sn or its compound,layers of the layer group 63 contain 38Sr or its compound and layers ofthe layer group 64 contain 37Rb or its compound. In this embodimentelements 51Sb and 38Sr play the same role as in the embodiment accordingto FIG. 2, that means that 51Sb is matched to the component 47Ag of theX-ray film and 38Sr to the component 35Br.

The role of the elements 50Sn and 37Rb is in producing a radiation forthe component 47Ag and 35Br, respectively, the energy level of which islower than the corresponding absorption energy level K, however it isadvantageously higher than the corresponding ⊕1 energy level. In such amanner atoms 47Ag and 35Br are pre-excited in order to obtain a betterefficiency with respect to the absorption of the radiation emitted by51Sb and 38Sr, respectively. So e.g.

    ______________________________________                                        50Sn             →                                                                            47Ag                                                   K = 29.20              K = 25.51                                              α1 = 25.27       α1 = 22.16                                       α2 = 25.04       α2 = 21.99                                       ______________________________________                                    

It can be well seen that the characteristic radiation of 50Sn is lyingbetween the absorption and emission energy levels of 47Ag. 48Cd and 49Inand their compounds are suitable pre-exciting elements to 47Ag.Characteristic radiation emitted by 37Rb performs pre-exciting of 35Br.

In the embodiment according to FIG. 3 before the first layer group 61layers 18 with 82Pb material are arranged for absorbing the scatteredradiation coming from the object examined and the environment, as wellas for the simultaneous emission of secondary radiation. Behind thetarget 7 layers 19 with 82Pb material are arranged for absorbing there-scattered radiation. Immediately before the target 7 one or morelayers 20 containing 82Pb are arranged for absorbing the scatteredradiation arising in the layer groups 61, . . . 64.

The embodiment according to FIG. 3 can be well used for radiographsserving for industrial purposes. In this case total thickness of thelayers 18 amounts to 100 to 500 μm, while this layer group is composedof 82Pb layers of the thickness of 1 μm or even less. Layers 19 can besimilarly formed. The single layer 20 may be a 82Pb layer of thethickness of 10 to 30 μm. Layer groups 61, . . . 64 are of the thicknessof 25 to 125 μm each, being formed of layers of 51Sb, 50Sn, 38Sr and37Rb, respectively, in a thickness of 1 μm or even less. Betweenadjacent layers e.g. Al₂ O₃ separating layers of 0.1 μm are to be found.Any element of the series can form a separating layer, so e.g. in thelayer group 61 separating layers of 50Sn of the thickness 0.1 μm can beused between the layers of 51Sb. For the sake of order it should bementioned that the thickness of the layer groups 61, . . . 64 need notbe necessarily identical, within one layer group layers may havedifferent thicknesses.

The modifying body according to the invention can be realizeddifferently from the embodiments shown in the drawings. So e.g. it canbe prepared as a flexible sheet, which is well matching to targetshaving not a plain surface.

I claim:
 1. A method for influencing the effect of X-ray or gammaradiation on a target sensitive to the radiation, the method comprisingthe steps of: irradiating the target with an X-ray or gamma radiationbeam and disposing a modifying body in front of and adjacent to thetarget in the path of the radiation beam, wherein a transmissiondirection is defined along the path from the modifying body to thetarget, and wherein said modifying body comprises at least twocontiguously disposed layer groups, each of said layer groups (e.g., 32)comprising a plurality of superposed metallic layers and separatinglayers therebetween, said metallic layers being provided for emitting asecondary radiation under the influence of the X-ray or gamma radiation,or the secondary radiation of the metallic layers in a previous layergroup (e.g., 31) with respect to the transmission direction,respectively, the energy of the secondary radiation emitted by saidmetallic layers being above the absorption energy level defined by theelectron shell K of an element being present in the subsequent layergroup (e.g., 33) with respect to the transmission direction, or in thetarget, respectively.
 2. The method as claimed in claim 1, wherein saidmodifying body further comprises at least one further layer group (62,64) emitting a secondary radiation, the energy of which is lying betweenthe absorption energy level and the emission energy level defined by theelectron shell K of an element being present in the metallic layers ofthe subsequent layer group with respect to the transmission direction,or in the target, respectively.
 3. The method as claimed in claim 1 or2, wherein said superposed metallic layers in each layer group contain ametal element with a lower atomic number than that of a metal elementbeing present in the superposed metallic layers in the previous layergroup with respect to the transmission direction.
 4. The method asclaimed in claim 1 or 2, characterized by further disposing saidmodifying body between two conductive layers and generating an electricfield between said two conductive layers, the direction of the fieldbeing substantially parallel to the transmission direction.
 5. Themethod as claimed in claim 1, wherein said modifying body comprises atleast three of said layer groups.
 6. A modifying body for influencingthe effect of X-ray or gamma radiation on a target sensitive to theradiation, wherein a transmission direction is defined in a directioncorresponding to that of the X-ray or gamma radiation travelling throughsaid modifying body, said modifying body comprising at least two layergroups disposed contiguously, each of said layer groups comprising aplurality of superposed metallic layers and separating layers disposedtherebetween, said metallic layers being provided for emitting asecondary radiation under the influence of the X-ray or gamma radiation,or the secondary radiation of the metallic layers in a previous layergroup with respect to the transmission direction, respectively, theenergy of the secondary radiation being above the absorption energylevel defined by the electron shell K of an element being present in themetallic layers of the subsequent layer group with respect to thetransmission direction, or in the target, respectively.
 7. The modifyingbody as claimed in claim 6, further comprising at least one furtherlayer group comprising a plurality of superposed metallic layers andseparating layers therebetween, said metallic layers emitting asecondary radiation, the energy of which is lying between the absorptionenergy level and the emission energy level defined by the electron shellK of an element being present in the metallic layers of the subsequentlayer group with respect to the transmission direction, or in thetarget, respectively.
 8. The modifying body as claimed in claim 6 or 7,wherein the thickness of each of said metallic layers in said layergroups is less than 30 μm.
 9. The modifying body as claimed in claim 6or 7, wherein the thickness of each of said metallic layers in saidlayer groups lies in the range between 0.1 and 10 μm and the thicknessof each of said layer groups lies in the range between 25 and 125 μm.10. The modifying body as claimed in claim 6 or 5, further comprisingone or more layers or coatings consisting of graphite or carbon orcarbon compound, producing Compton-electrons.
 11. The modifying body asclaimed in claim 6 or 7, wherein a surface of at least one of saidmetallic layers in at least one of said layer groups is provided with asubstantially monoatomic coating consisting of an alkali metal oralkali-earth metal or the oxides thereof or any other compound thereof,facilitating electron departure.
 12. The modifying body as claimed inclaim 6 or 7, further comprising two electrically conductive layers, andmeans for electrically insulating said two layers from the other layersin said layer groups, wherein two conductive layers are disposed so asto border said layer groups, each of said conductive layers beingprovided with an electric contact and a terminal connected thereto. 13.The modifying body as claimed in claim 6 or 7, wherein each of saidlayer groups is applied onto a carrier sheet which is substantiallytransparent to X-ray or gamma radiation.
 14. The modifying body asclaimed in claim 6 or 7, wherein all of said layer groups are appliedonto one single carrier sheet which is transparent to X-ray or gammaradiation.
 15. The modifying body as claimed in claim 13, wherein saidcarrier sheet is made of paper or plastic or aluminum foil.
 16. Themodifying body as claimed in claim 13, wherein said metallic layers andsaid separating layers in each of said layer groups are applied ontosaid carrier sheet by vacuum deposition.
 17. The modifying body asclaimed in claim 6, wherein said target includes an AgBr film, and theenergy of the secondary radiation emitted by the metal element of thelayer group which is last in the transmission direction is higher by amaximum of 100% than the absorption energy level of the element of thetarget.
 18. The modifying body as claimed in claim 6 or 7, wherein saidtarget includes an AgBr film, a first one of said layer groups comprisesmetallic layers containing antimony or its compound, a second one ofsaid layer groups comprises metallic layers containing strontium or itscompound and said second one of said layer groups is disposed after saidfirst one with respect to the transmission direction.
 19. The modifyingbody as claimed in claim 18, including a third one of said layer groupswhich third one comprises metallic layers containing at least one of theelements selected from a group of cadmium, indium and tin or at leastone of their compounds, wherein said third one of said layer groups isdisposed between said first and second layer groups.
 20. The modifyingbody as claimed in claim 19, including a fourth one of said layer groupswhich fourth one comprises metallic layers containing rubidium or itscompound, wherein said fourth layer group is disposed after said secondlayer group with respect to the transmission direction.
 21. Themodifying body as claimed in claim 6, wherein said modifying bodycomprises at least three of said layer groups.
 22. The modifying body asclaimed in claim 6, wherein said energy of the secondary radiation ishigher by a maximum of 50% than said absorption energy level.
 23. Amethod for recording an image of an object formed on a target by X-rayor gamma radiation, said method comprising the steps of: irradiating thetarget by directing an X-ray or gamma radiation beam through the objectand disposing a modifying body between the object and the target,adjacent to the target, in the path of the radiation beam, wherein atransmission direction is defined along the path from the object to thetarget, and wherein said modifying body comprises at least twocontiguously disposed layer groups, each of said layer groups comprisinga plurality of superposed metallic layers with separating layerstherebetween, said metallic layers being provided for emitting asecondary radiation under the influence of the X-ray or gamma radiation,or the secondary radiation of the metallic layers in a previous layergroup with respect to the transmission direction, respectively, theenergy of the secondary radiation being above the absorption energylevel defined by the electron shell K of an element being present in themetallic layers of the subsequent layer group with respect to thetransmission direction, or in the target, respectively.
 24. The methodas claimed in claim 23, wherein said modifying body further comprises atleast one further layer group comprising a plurality of superposedmetallic layers and separating layers therebetween, said metallic layersbeing provided for emitting a secondary radiation, the energy of whichis lying between the absorption energy level and the emission energylevel defined by the electron shell K of an element being present in themetallic layers of the subsequent layer group with respect to thetransmission direction, or in the target, respectively.
 25. The methodas claimed in claim 23 or 24, wherein said superposed metallic layers ineach layer group contain a metal element with a lower atomic number thanthat of a metal element being present in the superposed metallic layersin the previous layer group with respect to the transmission direction.26. The method as claimed in claim 23 or 24, characterized by furtherdisposing said modifying body between two conductive layers andgenerating an electric field between said two conductive layers, thedirection of the field being substantially parallel to the transmissiondirection.
 27. The method as claimed in claim 23, wherein said modifyingbody comprises at least three of said layer groups.